Method of making sulphite pulp



Dec. 15, 1942. Q PORTER ETAL 2,305,493

METHOD OF MAKING SULPHITE PULP Filed Sept. 19, 1940 y l/fali'er H Swanson PatentedDec. 15, 1942 UNITED STATES PATENT OFFICE 2,305,493 IHETHOD OF. MAKING SULPHITE PULP Donald 0. Porter, Appleton, and Walter H. Swanson, Menasha, Wis., assignors to Paper Patents Company, Neenah, Wis.,

a corporation of Wisconsin Application September 19, 1940, Serial No. 357,358

' 6 Claims. (01. 92-11) Our invention relates, generally, to the producpressure or greatest head. And since, the solution of sulphite pulp, and it has particular relation to an improved method of blowing down of the cookers or digesters used therein.

In the sulphite process of making chemical wood pulp, wood chips are digested incookers under pressure by a solution of calcium acid sulphite, made by passing an excess of sulphur dioxide into a suspension of calcium hydroxide. These cookers or digesters range in height from 4 5 to 60 feet, and are several feet in diameter. A single cooker may hold as much as 50 tons of chips. At the end of the initial cooking period, usually from 4 to 6 hours, the pressure in the digesters is reduced, during what is termed the blow-down part of the cook, in order to recover as much sulphur dioxide as possible from the digester liquor. During the blowing down of the digesters, the sulphur dioxide is liberated from the liquor and recovered for use in making up the fresh acid charge for the next cooker batch. The digester liquor is continuously circulated through the digester during the cooking period, including the blowing down period, by a suitable external circulating system.

While from the viewpoint of minimizing pulp cost and chemical efliciency, it is necessary that a blow-down operation, with the accompanying recovery of the dissolved sulphur dioxide from the cooking liquor, shall precede the discharging of the digester contents, it has been found very difficult to maintain an adequate rate of circulation of the digester liquor during the blowing down period. This material reduction in circulation is attributed to the foamy or frothy condition of the liquor caused by the release or evolution of sulphur dioxide therefrom, thereby reducing the circulating capacity of the circulating pump. As a result of this reduced rate of circulation, and the substantial difference in pressure between the top and bottom portions of the digester body, substantial amounts of the recoverable component of the cooking liquor are retained in the lower part of the digester and lost. Moreover, since the blow down period ordinarily constitutes a substantial part of the entire cooking period, lack of proper circulation during this period often results in very uneven digestion of the pulp which may render substantial portions thereof unfit for use.

A higher rate of circulation of the digester liquor or contents during the blow-down period would result in a much greater sulphur dioxide recovery. This is due tothe fact that the top of the digester is the point of lowest pressure while bility of sulphur dioxide decreases with decreasing pressure, and conversely, increases with increasing pressure, it will be seen that the more often the liquid contents of the digester can be pumped to the top, the greater will be the recovery of sulphur dioxide. However, as stated, in the usual method of operation the optimum recovery conditions do not exist because the rate of circulation is substantially decreased during the blow-down period by reason of the foamy or frothy conditions. It has been found that this reduction in the rate of circulation often reduces by one-half the number of times any given portion of the liquor is returned .to the low pressure zone at the top of the digester.

The principal object of our invention is to provide a method of reducing the pressure or blowing down thedigesters in sulphite pulp production without substantially reducing the normal rate of circulation, whereby a much greater sulphur dioxide recovery is obtained with a more uniform cooking of the pulp. Briefly stated, this object is accomplished by reducing the pressure in an interrupted or step-wise manner, and will be described in detail hereinafter.

In the drawing: v

Fig. 1 is a set of curves which illustrate the prior art method of digester operation; and

Fig. 2 is another set of curves illustrating our improved method of operation.

In Fig. 1 of the drawing, the temperature, di-

gester pressure, heater steam pressure, and power curves for a typical forced circulated sulphite pulp cook carried out according to standard operating procedure are shown. The description of the present invention is mainly concerned with the digester pressure and power curves which are controlled as a separate phase of the process and are not dependent upon the temperature or heater pressure. The power curve represents the power input to the circulating pump and hence is proportionalto the rate of circulation of the liquor.

At the startbf the cook after the chips have been introduced and the digester closed, the

pressure in the digester is brought up to the "maximum cooking pressure as rapidly as posthe bottom of the digester is the point of highest sible by means of a high head pump, or so-called booster pump. This pump injects fresh acid from the acid storage tanks at a rate sufficient to make up for the acid volume lost due to chip penetration, and injects enough additional acid to bring about a rapid rise inthe digester pressure. After maximum pressure is reached, the injection of acid is stopped, and the pressure h ld constant by means of an automatic pressure controller until the blow-down of the cook or the recovery period is started. The automatic pressure controller operates to relieve the pressure in the digesterfrom time to time by bleeding off a portion of the gases therefrom so that the pressure is maintained at a predetermined constant value. In the particular example shown, in Fig. 1, the digester pressure curve shows that one-quarter of an hour was consumed in bringing the digester pressure up to the maximum.

During the first five hours of the cook the digester pressure is maintained substantially constant after which the recovery or blow-down part of the cock is started. It will be understood that the recovery period may be started at any time itis deemed advisable by the operators, and that the five hour period is not critical. Usually, the blow-down period is commenced between the fourth and sixth hours. After the initial part of the cook, the digester pressure is reduced at a uniform rate from the maximum to as low a point as possible, as shown by the sloping portion of the digest-er pressure curve. In the example shown, this blow-down period consumes nearly four hours. At the end of the blow down period, the relief valve is'closed and the digester presleased during the blow down period.

sure raised preparatory to blowing the digester contents into the blow pit. This phase (discharging the digester contents) is shown by the portion of the digester pressure curve to'the right of the ninth hour ordinate.

The digester pressure schedule determines in a large degree the rate of circulation of the digester liquor, and hence the power input to the circulating pump. It will be seen that during the first five hours of the cook in which the digester pressure is maintained constant, the power curve also remains fairly constant. However; when the start of the blow down period occurs, there is a drop in the power input which coincides with the reduction of the digester pressure, and this power reduction continues until the lowest point in the pressure schedule is reached. This means that during the blow down period the rate at which the liquid contents are circulated is substantially reduced. Actually, in the example shown, there is about a 50% reduction in the rate of circulation during the blow down period.

Th's marked reduction in the circulation of the digester liquor during the blow down period has been accounted for by the frothy or foamy condl-ti-on of the liquor occasioned by the rapid re lease of sulphur dioxide. During the first four hours of the cook, there is only a small amount of sulphur dioxide and water vapor released from the digester, and'this is caused by the rising temperature of the digester contents which expand the liquid and decreases the amount of sulphur dioxide which can be held in solution. From the fourth to the fifth hour, there is even less gas released as during this period the temperature andpressure conditions remain substantially constant. However, at the fifth hour the somewhat sudden decrease in digester pressure causes a rapid release or evolution of sulphur dioxide'to an extent sufficient to cause a foamy or frothy condition 'of the liquor. Under this condition, there is a corresponding decrease in th power consumed by the circulating pump motor for foam 'In Fig. 2 of the drawing, the temperature, digester pressure, heater steam pressure and power curves for a cook or batch carried out under the improved method of operation of the present invention are shown. It will be seen that in this new procedure the digester pressure schedule and power curve up to the start of the recovery or blow down period are substantially the same as for a standard cooking operation as described above in connection with Fig. 1. However, during the blow-down period in our improved method of operation, instead of reducing the pressure steadily at a uniform rate until the lowest pressure is reached, we reduce the pressure in a series of interrupted steps with a holding between each step down. The rate of pressure drop during each step is somewhat greater than the rate of pressure reduction in the continuous steady drop of the prior art. In order to accomplish this stepwise pressure reduction the pressure controller cam of the relief valve is shaped to drop the pressure as rapidly as possible for a period of time equal to about the time it takes to circulate onethird of the digester liquid contents during the earlier portion of the cook, and then, the pressure is held constant for a similar length of time. In th particular example shown, there are five of these step-downs, in which the alternate pressure reduction and holding periods average about 10 to 12 minutes each. It will be understood that this particular rate of reduction is not critical but that it may be varied within limits as conditions require.

It will be seen that this method of reducing the pressure during the blow-down period has a marked eflect on the power input curve. Each time the pressure is reduced rapidly there is a corresponding marked decrease in power consumption, while during each time the pressure is held constant the power consumption increases very close to normal. This part of the power curve takes the form of a series of alternate high and low interconnected fiat sections. The lower flat portions of this part of the power curve indicate power input rates, and in turn circulation rates, considerably above corresponding power input and circulation rates obtainable in standard prior art operation. And the higher flat portions of this part of the power curve show that during a large part of the blow-down period the power consumption is only slightly below normal, thereby indicating nearly full circulation rates during this time. The average power consumption during .the entire blow-down period is much greater than the corresponding average in the standard method of operation outlined above in connection with Fig. 1.

It is not necessary that this stepwise or interrupted manner of pressure reduction be continued until the minimum blow-down pressure at the end of the blow-down period is reached.

does not circulate as rapidly as liquid, and hence tion is to decrease the rate of turn-over of the Referring to the power curve in Fig. 2, it will be seen that after a holding period at which the pressure is about 48 lbs/sq. in., the pressure is thereafter steadily-reduced to the minimum at a uniform rate. Different conditions of operation may require somewhat different variations in carrying out our improved method of blowing down the digesters.

At the end of the blow-down period in Fig. 2, the relief valve is closed and the digester pressure raised preparatory to blowing the digester contents into the blow pit' in the usual manner.

- greater liberation of sulphur dioxide.

Since the reduction in circulation during the blow-down period in our improved method of operation is substantially less than the reduction in circulation occurring in the previous standard method of operation, the rate at which the digester liquor is turned over in the respondingly greater. Accordingly, the liquid contents of the digester are brought to the top of the digester more often, thereby making for a This follows, since the greater the time the liquid is allowed to be in the low pressure zone at the top the greater will be the evolution of the dissolved sulphur dioxide.

Actual practice has shown that in operating a digester system according to the improved method of this invention approximately 35 more pounds of. sulphur dioxide can be recovered per ton of sulphite pulp produced than was heretofore recovered with standard methods of opera"- tion. This increased sulphur dioxide recovery represents a considerable saving in ;chem.ical

'cost and a corresponding reductionin pulp cost.

Sinc'e certain changes maybe made in the foregoing technique or method of operation of this invention without departing from the scope thereof, it'is intended that all matter described hereinbefore and shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense, and that the appended claims be given the broadest construction consistent with the prior art.

We claim the following as our invention:

1. The method of making sulphite pulp which comprises digesting wood chips in acid sulphite digester is corliberating sulphur dioxide from the digester liquor, continuing the circulation of the digester liquor during said blowing down of the digester, and blowing out the digester contents at the end of the cook, said blowing down of the digester in a step-wise manner serving to substantially reduce the normal drop in liquor circulation during the blow-down period and thereby permitting a substantially greater than normal sulphur dioxide recovery from the digester liquor.

3'. In the process of making sulphite pulp the steps which comprise, charging a digester with wood chips, injecting acid sulphite'solution into said digester at a rate suflicient to make up for acid volume lost due to chip penetration and enough additional to rapidly bring the digester pressure to the predetermined cooking pressure,

heating the liquor in the digester to cooking temgester pressure in an interrupted manner, re-

' the liquor during solution at elevated temperature and pressure with forced liquor circulation, and reducing the pressure after an initial cooking period in an interrupted stepwise manner to recover sulphur dioxide therefrom, the forced circulation of the liquor being continued during said pressure reduction, andsaid interrupted pressure reduction substantially reducing foaming of the-liquor and thereby permitting a relatively high rate 'of circulation and substantially increased oxide recovery.

' 2. In the process of making sulphite pulp the steps which comprise, charging a digester with wood chips, injecting acid sulphite solution into the digester thereby raising the pressure in the.

digester to cooking pressure, heating the digester charge, circulating the digester liquor, blowing down said digester in a stepwise manner thereby sulphur dimethod covering the sulphur dioxide evolved from the digester liquor during the pressure reduction or blow-down period, continuing the circulation of said blowing down of the digester, and blowing the contents from the digester at the end of the cook, said interrupted pressure reduction or blow-down serving to substantially reduce the normal drop in liquor circulation during the blow-down period and thereby permitting substantially increased recovery of sulphur dioxide from the digester liquor.

4. In the process of making sulphite pulp, the of substantially reducing the normal drop in liquor circulation during the blowing down of a digester which comprises reducing the pressure in an interrupted stepwise manner.

5. In the process of making sulphite pulp, the

" method of substantially reducing the normal drop in liquor circulation during the blowing down of a digester which comprises alternately reducing and holding the pressure.

6. In the'process of making sulphite pulp, the method of substantially reducing the normal drop in liquor circulation during the blowing down of a digester which comprises alternately reducing and holding the pressure, the reducing and holding periods being substantially equal.

DONALD C. PORTER. WALTER H. SWANSON.

the liquor during the initial 

